In a typical motor vehicle having an internal combustion engine and an electric starter motor according to the prior art, the operator of the vehicle cranks the engine by turning a key or pressing a button that closes an ignition switch. When the ignition switch closes, electric current is provided to the windings of an electric starter motor solenoid. Upon excitation of the solenoid, a plunger rod carried within the solenoid is caused to move in a linear direction. A linking rod connects one end of the plunger rod to the starter motor's pinion gear drive shaft. As the plunger rod moves, it causes the linking rod to rotate about a pivot point. Rotation of the linking rod about the pivot point moves the pinion gear drive shaft in a linear direction toward the flywheel ring gear of the motor vehicle engine.
Upon reaching the ring gear, the teeth of the pinion gear are designed to mesh with the teeth of the ring gear. To encourage full engagement of the pinion gear teeth and the ring gear, a small amount of axial rotation may be provided to the pinion gear as it moves toward the ring gear. Such rotation may be imparted, for example, with a helical spline gear positioned on the drive shaft of the electrical motor. The starter motor contacts then are closed and electric current is provided to the windings of the electric motor, causing the drive shaft of the electric motor to rotate the pinion gear. If the pinion gear teeth are engaged with the ring gear, rotation of the drive shaft and pinion gear causes the ring gear to rotate and crank the automobile engine.
A problem exists in with such a prior art starter motor. When the starter motor contacts are closed, a high inrush current from the battery or other power storage device causes the rotation of the starter motor drive shaft and pinion gear to accelerate rapidly. If there is any misalignment between the teeth of the pinion gear and the teeth of the ring gear, the pinion gear and ring gear may abut instead of meshing together. The rotation of the pinion gear may encourage the teeth to engage, but this too often is not the case if the pinion gear immediately begins rotating at a high rate. Instead, if the pinion gear teeth and the ring gear teeth are not enmeshed deeply enough when the electric motor transmits torque through the starter motor drive shaft, the pinion gear teeth can mill against the ring gear teeth rather than starting the engine. This also can cause damage to the starter motor and the ring gear.
Accordingly, it would be desirable to provide an anti-milling system for automotive starters. Such a system will promote the full engagement of the teeth of the starter motor pinion gear and the teeth of the ring gear prior to the acceleration of the starter motor drive shaft.